Conventionally, a liquid crystal display device has been provided with a plurality of pixel electrodes which are disposed in a matrix form, and opposing electrodes which oppose the pixel electrodes and serve as common electrodes. Liquid crystal serving as a display medium exists between the pixel and opposing electrodes. When a display is provided, a potential is selectively written by the pixel electrodes, and a voltage difference between the pixel electrodes and the opposing electrodes allows the liquid crystal existing therebetween to be subjected to an optical modulation, so that a display pattern is visually observed.
An active-matrix driving method has been known as a method for driving the pixel electrodes disposed in a matrix form. Each of the pixel electrodes is connected with a switching element and is driven by the switching element. A TFT (thin-film transistor) and an MIM (metal-insulating film-metal) elements are generally used as the switching element.
An active-matrix liquid crystal display device includes an active-matrix liquid crystal display panel, in which: (a) an active-matrix substrate having a plurality of scanning lines and a plurality of signal lines disposed so as to intersect one another on a transparent insulating substrate, each of the intersections having a pixel electrode and a switching element for driving the pixel electrode, and (b) an opposing substrate having opposing electrodes formed on a transparent insulating substrate, are provided with alignment films on the respective opposing surfaces and are bonded to each other via a liquid crystal layer.
FIG. 15 shows a construction of one pixel of the active-matrix substrate, which uses the TFT element (hereinafter, abbreviated as TFT) as the switching element.
A scanning line 2 is connected with a gate electrode of a pixel TFT 101, and a scanning signal inputted therein drives the pixel TFT 101. A signal line 3 is connected with a source electrode of the pixel TFT 101 and a display signal (video signal) is inputted therein. A drain electrode of the pixel TFT 101 is connected with a pixel electrode 104 and one of the terminals of an auxiliary capacity via an auxiliary capacity electrode 108. The other terminal of the auxiliary capacity is connected with an auxiliary capacity wire 4. Upon constructing a liquid crystal cell, the other terminal is connected to the opposing electrode disposed on the opposing substrate. On the insulating substrate, the pixel TFTs 101 and the pixel electrodes 104 are disposed in a matrix form.
FIG. 16 shows an example of the cross section of the active-matrix substrate. On an insulating substrate 117, a gate electrode 118, a gate insulating film 119, a semiconductor layer 120, an n.sup.+ -Si layer 121 serving as source and drain electrodes, a metal layer serving as a signal line 3, a between-layer insulating film 123, and a transparent conductor layer serving as a pixel electrode 104 are successively formed. The pixel electrode 104 is connected with the drain electrode of the pixel TFT 101 via a contact hole 125 penetrating the between-layer insulating film 123, specifically, via the auxiliary capacity electrode 108.
In the construction of FIG. 16, the between-layer insulating film 123 is formed between the scanning line 2 (disposed on the same layer as the gate electrode 118) and signal line 3 and the pixel electrode 104; thus, it is possible to allow the pixel electrode 104 to overlap the signal line 3. Such a construction makes it possible to improve an aperture rate and to shield an electric field caused by the signal line 3; consequently, alignment defects are prevented in liquid crystal.
Next, referring to FIG. 17, a succeeding process is described. FIG. 17 is a plan model view of the conventional active-matrix liquid crystal display device. Here, FIG. 17 illustrates a state in which one cell is cut out so as to correspond to each of the display devices of a large substrate. Actually, in many cases, a collective substrate is manufactured so as to include several cells formed laterally and longitudinally.
A polyimide alignment film is formed on an effective display area (inside a phantom line) 167 of a completed active-matrix substrate 150, and an aligning function is added by using an operation such as a rubbing operation. on an opposing substrate 151 as well, transparent opposing electrodes (not shown) including ITO (indium tin oxide) are formed, and then, a part corresponding to the effective display area 167 is subjected to the same operation.
Around the liquid crystal display panel, except for an inlet for filling liquid crystal (not shown), a sealing material (not shown) is applied thereon by using a printing method so as to surround the panel. Further, a conductive material 159 is applied to an opposing substrate signal input terminal 157 disposed on the active-matrix substrate 150, and then, spacers (not shown) are dispersed for maintaining a cell thickness of the liquid crystal layer, the active-matrix substrate 150 is bonded to the opposing substrate, and a heating operation is performed so as to harden the sealing material.
Afterwards, the cells, which are formed laterally and longitudinally in the collective substrate of the active-matrix substrate 150, are cut out one by one, liquid crystal is filled from the liquid crystal inlet, and the liquid crystal inlet is filled with the sealing material so as to achieve the panel of the liquid crystal display device. And then, a source driver 160a for applying a display signal to each of the signal lines 3, a gate driver 160b for applying a scanning signal to each of the scanning lines 2, a control circuit (not shown), a back light (not shown), and other members are installed so as to complete the liquid crystal display device. Here, the liquid crystal display device of FIG. 17 is not provided with the auxiliary capacity wire 4.
Normally, on such a liquid crystal display device, an optical inspection is performed in each step of the process, an electrical inspection is performed when the active-matrix substrate is completed, and a lighting and an electrical inspections are performed between when the panel is completed and when members including a driver have been installed.
These inspections are carried out so as not to allow defected portions to remain in the succeeding process. The defected portions cause loss of materials and works. When a defect is found, the substrate is discarded immediately, or the defect is corrected by using a means such as a laser.
Incidentally, because of the recent improvement of manufacturing techniques, a liquid crystal display panel has offered a higher definition. Accordingly, the inspection also requires a more improved technique.
Namely, terminals for inputting signals to the signal lines 3 and the scanning lines 2 has a shorter pitch. When pins electrically become contact with the lines so as to supply signals, it is necessary to provide an extremely fine and expensive probe. Further, in some cases, even small dust on the terminal does not allow a normal lighting upon inspection; thus, an inspection defect may be mistakenly recognized as a defect of the panel. In order to solve this problem, the inspection needs to be performed in an extremely clean environment. Therefore, it is necessary to increase the total cost.
Hence, in order to realize a simpler lighting inspection performed upon completion of the panel, in the liquid crystal display device of FIG. 17, the signal lines 3 and scanning lines 2 for applying the same signals upon inspection are respectively short-circuited by inspection display signal lines 152a, 152b, and 152c, and inspection scanning signal lines 153a and 153b. Normally, after the inspection, these lines are cut of together with the substrate portion by dicing along cutting lines L1, or they are electrically cut off by using a means such as a laser cutting (for example, see Japanese Published Unexamined Patent Application no. 005481/1995 (Tokukaihei 7-005481, published on Jan. 10, 1995)).
However, this method causes the following problem: the number of the steps is increased by including the cutting operation. Further, another defect may occur due to a flake of the wiring pattern or a fragment of glass during the cutting operation. Moreover, in the case of the dicing operation, a large substrate needs to be provided with useless areas to be cut out, resulting in a decrease in the number of panels.
Therefore, in order to solve these problems, as shown in FIG. 18, inspection signal lines for applying the same inspection signal are not electrically continuous in a complete manner. Instead, active elements such as TFT are placed and a signal is applied upon inspecting if necessary so as to bring the active element into conduction; thus, it is possible to obtain the same effect as the arrangement having a short circuit. This method is disclosed in Japanese Published Unexamined Patent Application no. 333275/1995 (Tokukaihei 7-333275, published on Dec. 22, 1995). In the liquid crystal display device, the signal lines 3 and scanning lines 2 for applying the same signals upon inspection are respectively short-circuited by inspection display signal lines 172a and 172c, and inspection scanning signal line 173a. Further, inspection TFTs 174a and 174b are formed respectively for the signal lines 3 and the scanning lines 2, and switching wires 172b and 173b are formed for entering a signal which turns the TFTs on and off.
Incidentally, as described above, it is more cost-effective to find a defect appearing in the process as soon as possible and to correct the defect or to discard the substrate. Especially, after the active-matrix substrate is completed, the total cost of the product considerably varies depending upon whether an expensive color filter is bonded to or not, and whether an extremely expensive liquid crystal material is filled or not, before a defect has been found on the substrate. Therefore, more than the optical inspection, on the completed active-matrix substrate, it is critically important to carry out an electrical inspection, in which an inspection signal similar to an actual driving signal is written via the active element to the pixel, and the inspection signal is read therein.
Here, another method is known as an electrical inspection method, in which signals are successively applied via the active elements on a large-format substrate, each of the pixels is charged, and then, the signals are successively read via the active elements so as to electrically obtain information on a defect occurring on the screen (for example, see Japanese Published Unexamined Patent Application no. 142499/1991 (Tokukaihei 3-142499, published on Jun. 18, 1991)).
This method makes it possible to detect a defected active-matrix substrate before the opposing substrate has been bonded. However, when a point defect is detected by using this method, it is necessary to precisely read an extremely small electrical signal. Thus, in addition to difficult problems such as the arrangement of a reading amplifier, optimization of the circuit sequence, and a balance between the time constant and the reading time period of the pixel TFT, this method causes a large difference between the electrical inspection result and the display inspection result, especially upon detecting irregular display and a low-brightness point.
Furthermore, in another method, materials, whose refractive indexes vary in accordance with electric fields, are placed around the pixel electrodes, and the materials are successively scanned so as to detect a defect entirely on the substrate by reading a potential of each of the pixels through a behavior upon emitting light on these materials (for example, see Japanese Published Unexamined Patent Application no. 240800/1993 (Tokukaihei 5-240800, published on Sep. 17, 1993)).
This method makes it possible to eliminate influence of noise when a signal is read via the active element and the inspection wire, so that accuracy of the inspection can be improved. Furthermore, as described in the method of Tokukaihei 7-005481, in the case when the wires, which apply the same signals at a lighting inspection after the panel has been completed, are previously short-circuited by the inspection signal lines and the inspection scanning lines, the wires can be used upon an electrical inspection of the process.
However, as for all the above-mentioned methods, upon inspection on a large-format substrate, it is necessary to apply a plurality of kinds of signals to each of the cells. The smaller the cell is, or the more individual cells are disposed on a large format, it becomes more difficult to apply a signal to each of the cells.
In general, a probe frame is used as a means for applying a signal to a cell. In the probe frame, the frame board having the same size as the substrate is provided with windows corresponding to the cells, and signal input pins are set up so as to surround the windows. In the active-matrix substrate opposing the probe frame, a vacant area, which surrounds an effective display area of each of the cells, is provided with signal input pads.
However, unlike a large model, vacant areas are small in a small model, so that a restriction is imposed on providing the signal input pads for inspection. Especially, in the case when an inspection wire is provided for each of RGB upon performing an electrical inspection for a color display in order to enhance accuracy of the inspection, or in the case when a scanning line of the adjacent pixel is used as an auxiliary capacity wire in the same manner as a so-called Cs On Gate structure so as to input a number of signals for driving one cell, the signal input pad needs to be smaller in size. For this reason, this arrangement further causes a poor contact of the pins in the probe frame and increases the cost of manufacturing the probe frame in accordance with a higher density.
Furthermore, in the case of the extremely small windows of the probe frame, a detection element cannot freely move inside the window, a ratio of ineffective area surrounding the element becomes larger in the cell, and in addition, the detection element may become larger than the window in the worst case; consequently, the detection element cannot come close to the pixel electrode, so that the inspection may become impossible.
Additionally, the more individual cells are placed on a large-format substrate, the number of the pins considerably increases. The number of the pins is determined by the number of positions .times. the number of kinds of signals. Generally, the cost of manufacturing the probe frame is determined by man-hours for opening the windows, the number of the disposed pins, and the accuracy; thus, if the more cells are disposed, all the other factors increase accordingly, resulting in a significant raise in the cost.